I would have tried querying these within AlphaDictionary but I haven't quite figured out how to do it because I know some of you usually paste older question/answers.
When did tis migrate to it's? Are these not the contraction of the same two words?

I think "It is" probably came before "Tis," and that perhaps "Tis" comes from hurrying the pronunciation of "It is" with an unvoiced "I" in "It" and a liaison of the "t" in "It" with the word "is" to avoid a full stop, as in "i tis." Everyone knows how lazy we English speakers are!

"It is" (both words fully pronounced)

"I-tis" ("I" fully pronounce but "t" moved to next word [liaison] to avoid a full stop)

"i-tis" ("i" unvoiced and "t" moved to next word [liaison] to avoid a full stop)

"Tis" (final contraction to new word)

Regards//Larry

"To preserve liberty, it is essential that the whole body of the people always possess arms, and be taught alike, especially when young, how to use them." -- Attributed to Richard Henry Lee

I didn't even know that it's an official word!
Which makes wonder why is it allowed when a word becomes famous enters the dictionary? Especially when it is a corrupted version of another word? or is this evolution of words?
Though it may not be common, but I think it's still there - or should I say 'tis? - and it would be good if anyone can mention similar change in words.

The current goodword may be considered the same.

Today's word has no commonly encountered relative but it is occasionally used as a verb, to somersault, which makes somersaulter a possibility. It is used metaphorically to indicate absolute joy in the phrase "to do somersaults". In several dialects this word has been further reduced to somerset by folk etymology. In some areas of the US South it has been combined with tumble to produce tumbleset with the same meaning.

"Do you want to know hell, it's depths; hell is the company of the ignorant." My translation for a verse by Omar Al-Khayyam.

noun: a word formed from two or more words by omitting or combining some sounds ("`won't' is a contraction of `will not'")

It apears that I have misunderstood the situation. Execuse my lack of knowledge.

Modi,

Don't worry. English may not be your native language, but you seem to have a very good grasp of it. Contractions are often used in conversational speech, and most are so frequently used that they are second nature to a native speaker. I don't know when (or how) they are taught when English is taught as a Foreign Language.

Here's a joke I like to tell:

What do you call a person who speaks three languages?

Trilingual.

What do you call a person who speaks two language?

Bilingual.

What do you call a person who speaks only one language?

An American!

Sad, but mostly true.

Regards//Larry

"To preserve liberty, it is essential that the whole body of the people always possess arms, and be taught alike, especially when young, how to use them."